1. Field of the Invention
The present invention relates to a projection exposure method and a projection exposure apparatus, which are usable for production, for example, of semiconductor devices, liquid crystal display devices, etc., and in which exposure is carried out by adjusting a photosensitive substrate adjusted so as to become coincident with the image-formation plane of a projection optical system by an auto-focusing method using a result of detection obtained by an oblique incidence type focus position detecting system (AF sensor). More particularly, the present invention relates to calibration of the oblique incidence type focus position detecting system.
2. Description of Background Art
In a projection exposure apparatus used to produce, for example, semiconductor integrated circuits, liquid crystal panels, etc., a photosensitive substrate must be adjusted so as to be exactly coincident with the image-formation plane of a projection optical system. Therefore, projection exposure apparatuses have heretofore been provided with an auto-focus mechanism for adjusting the photosensitive substrate to the image-formation plane of the projection optical system by controlling the height of the wafer stage on the basis of the position of the photosensitive substrate in the direction of the optical axis of the projection optical system, which is detected by using an oblique incidence type focus position detecting system (hereinafter referred to as "AF sensor").
The oblique incidence type AF sensor comprises a light-transmitting system for projecting a slit pattern or other image onto a predetermined measuring point on a workpiece, for example, a photosensitive substrate, obliquely to the optical axis of the projection optical system, and a light-receiving system which receives reflected light from the photosensitive substrate, re-forms the slit pattern or other image, and detects an amount of deviation of the position where the image is re-formed from a reference image-formation position. In this case, a position at which the slit pattern or other image is re-formed when the photosensitive substrate is coincident with the image-formation plane of the projection optical system (i.e. the reference image-formation position) has previously been obtained. Then, in actual process, the height (i.e. the position in a direction perpendicular to the image formation plane of said projection optical system) of the photosensitive substrate is adjusted so that the position where the slit pattern or other image is re-formed coincides with the reference image-formation position, thereby effecting auto-focusing for the photosensitive substrate.
As a device for directly measuring the position of the image-formation plane (i.e. focus position) of the projection optical system, a stage emission type focus position detecting system has heretofore been known. In this type of focus position detecting system, for example, an image of a predetermined reference pattern provided on a wafer stage is projected onto the lower surface (pattern surface) of a reticle as a mask through a projection optical system above the stage by means of light from an illumination system which is separate from an illumination system for exposure of the projection exposure apparatus. Then, reflected light from the reticle is returned toward the wafer stage through the projection optical system and received through the reference pattern. Thus, a position at which a detection signal, which is obtained as the wafer stage is vertically moved, assumes an extreme value is detected as the position of the image-formation plane.
There is also known a stage slit type focus position detecting system. In this type of focus position detecting system, for example, a reticle as a mask is illuminated by an illumination optical system of a projection exposure apparatus to project an image of a predetermined opening pattern unit for detection (having a plurality of slit-shaped transparent portions), which is specially provided on the reticle, onto a wafer stage through a projection optical system, and the image-forming light thus projected is received through an opening pattern for detection which is specially provided on the wafer stage. Thus, a position at which a detection signal, which is obtained as the wafer stage is vertically moved, assumes an extreme value is detected as the position of the image-formation plane.
As has been described above, the use of the oblique incidence type AF sensor makes it possible to accurately detect an amount of deviation of a measuring point on the photosensitive substrate from the detected position of the image-formation plane of the projection optical system. However, when exposure is continuously carried out, thermal energy of exposure light is accumulated in the projection optical system, causing the image-formation characteristics of the projection optical system to change. Accordingly, the detected position of the image-formation plane, which should essentially be fixed, may change. If the position of the image-formation plane changes in this way, the oblique incidence type AF sensor detects an amount of deviation of the measuring point on the photosensitive substrate from the image-formation plane (reference position) determined before the change of the position. Therefore, if auto-focusing is carried out by using a result of detection made by the oblique incidence type AF sensor, the photosensitive substrate cannot be adjusted so as to be coincident with the image-formation plane after the change of the position.
With the stage emission type focus position detecting system, even when the position of the image-formation plane of the projection optical system has changed because, for example, of thermal deformation of the projection optical system caused by absorption of thermal energy of exposure light, the position of the image-formation plane of the projection optical system can be accurately detected. However, the stage emission type focus position detecting system uses an illumination optical system which is independent of the illumination optical system for exposure of the projection exposure apparatus. Accordingly, the stage emission type focus position detecting system suffers from the problem that there may be a predetermined offset between the position of the image-formation plane of the projection optical system when the reticle is actually illuminated with exposure light from the illumination optical system for exposure of the projection exposure apparatus, and the focus position measured by the stage emission type focus position detecting system using light from the independent illumination optical system.
In regard to the illumination optical system for exposure, there are cases where a modified light source illumination method, an annular illumination method, etc. are selectively used, as disclosed, for example, in U.S. patent application Ser. No. 791,138, in order to increase resolution. If illumination optical systems are changed over from one to another as described above, the exposure light illuminance distribution on the pupil plane (Fourier transform plane) of the projection optical system changes, and the position of the image-formation plane of the projection optical system also changes, resulting in a change of the offset between the focus position measured by the stage emission type focus position detecting system and the position of the image-formation plane of the projection optical system when exposure light is actually applied.
The use of the stage slit type focus position detecting system makes it possible to detect the position of the image-formation plane of the projection optical system when exposure light is actually applied because this focus position detecting system uses the illumination optical system for exposure of the projection exposure apparatus. However, the stage slit type focus position detecting system has the problem that, since it is particularly necessary to use a special reticle formed with a predetermined opening pattern unit for detection, it is impossible to detect the position of the image-formation plane when a reticle for actual exposure is used. Even if the reticle used for actual exposure is provided with an opening pattern unit for detection, since the opening pattern unit can be provided in only a limited area other than the pattern area for exposure, it is difficult to measure the position of the image-formation plane at a desired measuring point in the exposure field of the projection optical system.
Further, neither of the stage emission and stage slit type focus position detecting systems can be used when the photosensitive substrate is set in the exposure field of the projection optical system because the reference (opening) pattern provided on the wafer stage is out of the exposure field at that time. Therefore, these focus position detecting systems cannot be used as AF sensors for auto-focusing.